Wood ash, a byproduct of woody biomass power generation, has potential as an alternative K fertilizer due to its high K content and pH-raising properties. However, concerns remain about heavy metal contaminants like Cr and the limited understanding of element dynamics in soil–solution–crop systems after wood ash’s application. This study examined the effects of 1% (w/w) wood ash on element dynamics and komatsuna (Brassica rapa var. perviridis) uptake in low-K soil through a pot experiment. XRD was used to analyze mineral composition, SEM-EDS to observe surface and elemental properties, and XANES to examine Cr speciation in wood ash. Soil solution analysis covered macro- and micronutrients, heavy metals, anions, pH, and DOC, while crop element concentrations and aboveground dry weight were also quantified. The chemical speciation of Cu and Cr in a soil solution was modeled using Visual MINTEQ. Wood ash significantly increased K concentrations (from 17 mg/L to 650 mg/L) in the soil solution, along with Ca, Mg, P, and Mo, while reducing Ni, Mn, Zn, and Cd levels. Komatsuna K uptake surged from 123 mg/kg to 559 mg/kg, leading to a 3.31-fold biomass increase. Notably, the Cd concentration in the crops dropped significantly from 0.709 to 0.057 mg/kg, well below the Codex standard of 0.2 mg/kg. Although Cu and Cr concentrations rose in the soil solution, crop uptake remained low due to >99% complexation with fulvic acid, as confirmed by Visual MINTEQ modeling. This study confirms that wood ash is an effective K fertilizer, but emphasizes the need for risk mitigation strategies to ensure safe and sustainable agricultural application.

Thallium (Tl) is a highly toxic metal that induces pathological changes in organs. Many Tl-contaminated soils have been reported worldwide due to the releases of Tl from mineral weathering and mining industry. There is, however, limited information related to the dynamics and analysis of Tl in soil, since Tl is one of the emerging soil contaminants. Tl and cobalt (Co) are specifically adsorbed by manganese (Mn) oxide. On the other hand, air-drying of soils causes the dissolution of Mn oxide and the elution of heavy metals such as Co occluded by Mn oxide. Thus, the Mn oxide dissolution with air-drying is expected to affect the chemical forms of Tl with high sorption affinity to Mn oxide. We conducted the sequential extraction method using moist and air-dried soils. Air-drying caused the increases in Mn and Co in the exchangeable and acid soluble fractions, respectively, and the decreases in those in the Mn oxide occluded fraction, which is consistent with previous reports. Our research revealed that the exchangeable Tl increased and sum of the acid-soluble and Mn oxide occluded Tl decreased due to air-drying. The increments in the exchangeable fraction and the decrements in acid-soluble and Mn oxide occluded fractions were almost the same. Consequently, the increase in exchangeable Tl could be attributed to the other two fractions, indicating that soil Tl is sensitive to the soil drying.

The efficient separation of solid and liquid phases of soil under reductive conditions is of the utmost importance to study soil chemistry and to predict the mobility and bioavailability of nutrients and toxic contaminants in waterlogged reduced soils (WRSs). However, there is no established method for efficiently separating the solid and liquid phases of WRS within a short time while maintaining its reductive conditions. This study aimed to verify the applicability of a simple centrifugal filtration method (CFM) for the efficient separation of solid and liquid phases of a WRS and examine the CFM-extracted soil solution to confirm that the reductive condition was maintained during the solid–liquid separation process. Incubation experiments were performed under reductive conditions with or without ethanol/molasses used as additional organic material (OM), while the soil solution was collected by both a suction method and CFM at different centrifugation speeds (700, 2760, and 11,000 rpm) and times (1–7 min). The results showed that the soil pH increased with time while the Eh decreased, indicating that its reducing state was enhanced during the incubation experiments. The addition of OM promoted the reductive conditions in the first days of the experiments. Centrifugation speed, rather than time, was found to be the key to extract the maximum amount of soil solution, while a higher centrifugation speed (11,000 rpm), which represents the permanent wilting point, was found to be most effective for extracting the maximum amount of soil solution. The results exhibited no significant difference in solute (As, Fe(II), and Mn) concentrations when varying amounts of CFM-extracted soil solution were measured. The statistical analysis also indicated no significant (p > 0.05) difference between the solute concentrations in the CFM-extracted soil solution and the solute concentrations in the soil solution extracted by the suction method, confirming that the reductive condition was maintained during solid–liquid separation by CFM. This study suggests that CFM operating at a higher centrifugation speed could potentially be employed as a simple and highly effective technique to efficiently separate the solid and liquid phases of WRS (sandy clay loam) within a short time while maintaining its reductive conditions.